Electronically scanned arrays (ESAs; sometimes referred to as phased array radars or phased array antennas) may be active ESAs (AESAs) or passive ESAs (PESAs). ESAs often include radiofrequency (RF) power amplifiers. Such RF power amplifiers receive RF power inputs and output amplified RF signals that are provided to antenna elements of such ESAs.
Currently implemented RF power amplifiers for ESAs can operate in a linear state or in a compression state (also referred to as saturation). When a currently implemented RF power amplifier operates in a linear state (which also may be referred to as a linear region), phase of the power amplifier output remains relatively constant and the RF power amplifier's amplitude increases proportionally to an increase in RF power input. Operating in a linear state typically results in poor efficiency (e.g., power added efficiency (PAE)).
When a currently implemented RF power amplifier operates in a compression state, the efficiency and phase increase as the RF output at the RF power amplifier increases. This effect is sometimes referred to as amplitude-phase distortion (“AM-PM distortion”) and may be measured in degrees per decibel (dB). AM-PM distortion is undesirable for communication systems and is especially undesirable for ESA systems.
Precise aperture phase and amplitude control are required for high performance AESAs. AM-PM distortion during compression can result in imprecise phase shift, which often results in reduced beam pointing accuracy. The amplitude and phase of transmitted power at each antenna element in an ESA is dependent on the RF input to the RF power amplifier. Any of various sources of error, such as from processes, materials, and assembly variations, can cause small disturbances in the RF input signal applied to an RF power amplifier that can result in a deviation from a desired amplitude and phase for a transmitted signal from the ESA. While operating an RF power amplifier at compression typically reduces the output amplitude's sensitivity to deviations in the RF input power and improves system efficiency, the phase is typically strongly dependent of the RF input power level. For example, a typical AM-PM distortion for a low power RF power amplifier in an ESA may be about 3-10 degrees/dB, which means every 1 dB change to an input signal would generate 3-10 degrees of phase distortion in the output of the RF power amplifier. At such a 3-10 degrees/dB distortion at saturation, a 0.5 dB change in the input would produce 1.5-5 degrees uncertainty in phase. Such phase uncertainty can broaden a beam width, thus reducing the beam pointing accuracy.